The present invention relates to plasma panels (PP), that is to say flat display screens in which the displayed image consists of a number of light-discharge points. The light discharges are produced in a gas contained between two insulating tiles, each point corresponding to an intersection in electrode arrays borne by at least one of the tiles.
The present invention relates more particularly to a process for the manufacture of barriers on at least one of the tiles of the panel, these barriers themselves being structural elements well known in the PP field.
It is known that a PP comprises a two-dimensional matrix of cells organized in rows and columns, which is traced to the geometry of the electrode arrays. In this case, the barriers are relief elements intended to separate the rows or the columns of cells. In some panels, the barriers may also separate both the columns and the rows of cells, therefore forming a chequerboard pattern of the latter. The role of the barriers is multipurpose. Thus, by partitioning the space of each cell at least in the direction of the rows or of the columns, the barriers prevent a discharge in one cell from inducing undesirable discharges in neighbouring cells by the ionization effect. They thus prevent cross-torque phenomena.
Moreover, the barriers constitute optical screens between the neighbouring cells, allowing good confinement of the radiation emitted by each cell. This role is particularly important in colour PPs in which the neighbouring cells constitute dots of different colours, in order to form triads for example. In this case, the barriers ensure good saturation of the colours.
Furthermore, the barriers often act as spacers between the tiles of the panel. Thus, the fact that the barriers may have a height corresponding to the required separation between the two tiles may be exploited. In this case, the tile not provided with barriers rests on the tops of the barriers present on the other tile.
The barriers may have various structures. However, if they are intended to be supporting, they are conventionally made of a dense and hardened material. These supporting barriers must be able to withstand the considerable pressure exerted by one tile on the other. This is because, during the operation of vacuum-pumping the space between the two facing tiles, prior to introduction of the low-pressure discharge gas, the force exerted per unit area of barrier may be as much as 106 pascals (10 kg/cm2), depending on the ratio of the area of the barriers to the total area of the panel. In the current state of the art, the barriers are composed of a dense material, generally a glassy phase, which is sufficiently crush-resistant to maintain a constant space between the two tiles. These barriers are produced, for example, by screen-printing (in 10 to 20 successive layers) a paste containing a glass frit or by blasting a layer containing a glass frit. After producing the geometry of the barriers, these layers are fired at temperatures of between 450xc2x0 C. and 600xc2x0 C. (typically 550xc2x0 C.) so as to densify the material and make it mechanically strong. However, the densified material always exhibits porosity throughout it and this porosity cannot be easily pumped during the operation of vacuum-pumping the panel, which lasts only a few hours (generally 4 to 15 hours at 150xc2x0 C. to 350xc2x0 C.). Even if this porosity is low, and even if the surface of the barriers is perfectly vitrified, outgassing may occur over the few tens of thousands of hours that constitute the lifetime of a plasma panel. Any contamination of the gas phase in a PP causes operational variations which may be manifested either in terms of the operating voltages or on the luminous efficiency or on their lifetime. To remedy this drawback, it has been proposed in French Patent Application No. 98/16093 in the name of Thomson Plasma to produce the barriers from a material giving them substantially open porosity, the porosity being advantageously also relatively high. For this purpose, the Applicant has discovered that if barriers with a high porosity are produced, it is possible to remove from them, during the vacuum pumping, practically all the molecules capable of outgassing, so that the risk of the panels subsequently outgassing hardly exists any more. This technical effect is all the more remarkable in that the duration of the vacuum-pumping step can be reduced from several hours to less than one hour, or even only thirty minutes, without the performance characteristics of the PP being affected thereby.
In patent application Ser. No. 98/16093, the barriers are produced by using conventional manufacturing processes, such as screen printing, blasting and photolithography. Thus, as illustrated in FIGS. 1a to 1c, the barriers are produced on a tile 1 having address electrodes X1, X2 . . . X5 . . . . For example, these barriers have, at the end of the manufacturing process, a 400 xcexcm pitch, a 100 xcexcm width and a 180 xcexcm height, for a plasma panel having a working area corresponding to a 106 cm diagonal with TV resolution (560 rows, 700 columns). In a known manner, a thick layer of dielectric 2 and a thin layer of magnesium oxide or MgO have been deposited using conventional techniques on the tile 1 covered with the address electrodes.
The barriers are produced by photolithography of a pasty layer 10xe2x80x2 deposited by screen printing on the thin MgO layer 3. The composition of the paste forming the layer is as follows:
a mineral filler in the form of alumina particles having a mean particle diameter of 5 microns with a narrow particle size distribution;
a glassy phase, which may be lead borosilicate or bismuth borosilicate at a level of 10% of the mass of the alumina and a photosensitive resin of the negative type, constituting 50% of the volume of the paste.
Using a doctor blade 20, the paste 10xe2x80x2 is spread uniformly over the MgO layer 3 through a screen-printing mask 21 having an aperture corresponding to the aspect ratio of the working area of the tile, as illustrated in FIG. 1a. The layer of paste 10xe2x80x2 is dried at 80xc2x0 C. After this operation, it has a thickness of about 20 xcexcm.
Next, a photolithography mask 22 is laid over the layer of paste 10xe2x80x2. The mask has an elongate-aperture pattern corresponding to the pattern of barriers to be printed on the MgO layer 3. Those parts of the layer which are revealed by the mask are exposed to ultraviolet radiation so as to make them resistant to the development, as illustrated in FIG. 1b. 
Next, the layer 10xe2x80x2 thus exposed is deposited in water or in water to which sodium carbonate has been added, depending on the type of resin used, and then the surface is dried using an air knife.
A first layer of barrier material 10xe2x80x2 with an elementary height of approximately 20 xcexcm is then obtained, as illustrated in FIG. 1c. 
The above steps are repeated in succession until the total required height for the barriers is obtained. Each new deposition of paste 10xe2x80x2, by screen printing, completely covers the working area of the tile, including the tops of the barriers being formed. Depending on the number of iterations of these steps, the vertical position of the screen-printing mask 21 or the depth of the latter is modified, depending on the variation in the deposited layers existing on the tile.
The process described in patent application Ser. No. 98/16093 requires several passes in order to be able to produce barriers having the required height. Typically, the process requires from 3 to 5 deposition operations since the individual thicknesses are small, of about 15 to 30 xcexcm. In order to deposit barriers with a height of 150 xcexcm, at least 5 layers are therefore required, with intermediate drying steps and a final firing at 400xc2x0 C. to 520xc2x0 C. for 20 to 60 minutes in order to stabilize the deposited structure and to burn off the organic compounds.
The object of the present invention is therefore to propose a process for the manufacture of the barriers which is much simpler and much more rapid.
As a consequence, the subject of the present invention is a process for the manufacture of a plasma panel comprising two tiles facing each other and containing a plasma discharge gas, at least one of the tiles having an array of electrodes serving to define a number of discharge cells and an array of supporting barriers delimiting the cells, the carriers being made of a material giving them a high and open porosity, characterized in that the barriers are formed in a single step using a paste comprising the said material and an organic resin.
Two standard processes may be used to manufacture the barriers in a single step, namely a moulding-type forming process or a transfer-type forming process.
According to a first mode of implementation, relating to the moulding-type forming process, this comprises the following steps:
deposition of a uniform layer of paste on the tile receiving the barriers;
application to the said layer of a mould having the pattern of barriers; and
printing, by pressing the pattern into the deposited layer.
In this case, the organic resin contained in the paste is a thermoplastic resin which has, preferably, a softening temperature of between 60xc2x0 C. and 200xc2x0 C.
Typically, this organic resin includes compounds chosen from polyvinyl alcohol, polyvinylpyrrolidone and polyvinyl butyrate. In addition, the resin represents from 25 to 70% of the total mass of the paste. Moreover, in this process, the pressing is carried out at a temperature of between 70xc2x0 C. and 150xc2x0 C.
According to another mode of implementation, relating to transfer-type forming, the process comprises the following steps:
filling a mould having the pattern of barriers with the said paste;
pressing the mould onto the surface of the tile receiving the barriers; and
adhesion of the paste by heating.
In this case, the organic resin contained in the paste comprises a curable compound which has a softening temperature of between 80xc2x0 C. and 150xc2x0 C., chosen from vinyl or cellulose compounds. In order to make the material of the paste adhere to that surface of the tile receiving the barrier, this surface is heated to a temperature of between 80xc2x0 C. and 150xc2x0 C.
The material of the barriers having a high and open porosity is identical in both modes of implementation. It is also identical to the material described in patent application Ser. No. 98/16093. Typically, this material includes a mineral filler in the form of a powder having a mean elementary particle diameter of between 1 and 20 xcexcm. Preferably, the mineral filler is an oxide chosen from alumina and silica.
The material of the barriers may optionally include a hardening agent in an amount equal to or less than 10% of the mass of the mineral filler. This hardening agent is a glassy phase with, in the case of a glass, a softening temperature of less than the treatment temperature. This glassy phase is chosen from lead borosilicate, bismuth borosilicate and compounds such as lead sulphate, lead phosphate, zinc phosphate, sodium silicate, potassium silicate, lithium silicate and lead silicate, these being capable of forming chemical bonds at the treatment temperature.
According to another characteristic of the present invention, after forming the barriers, phosphors are deposited between them, using a conventional deposition process such as a screen-printing or photolithography process.
Once the phosphors have been deposited, the tile bearing the barriers is then subjected to a final firing at a temperature of between 400xc2x0 C. and 500xc2x0 C., preferably between 400xc2x0 C. and 450xc2x0 C., so as not to deform the tile which is made of glass. This is because the dimensional stability of the glass is difficult maintain above 460xc2x0 C.